Slow light speed

I recall reading a story a couple decades ago - might have been written a decade or two before that.

In this book, a full-length novel, the Earth has been traveling for a long time (eons?) through a region of space where the speed of light is abnormally low. One day, it emerges from this region and suddenly the speed of light is much faster. People are thinking faster and becoming more intelligent - so are the animals.
People find that travel through the stars at speeds much faster than the former speed of light is now possible and most plan to leave the Earth. Others, formerly not the brightest bulbs, are depressed and artificially limit their intelligence.

Though the plot isn't ringing any bells, the concept of shifts that affect thinking and the speed of light sounds just like Vernor Vinge's zones of thought from 'A Fire Upon the Deep.' Perhaps a Vinge short story?